The ZEN15 Power Switch won't have time to send any information to your hub if power goes out, but it will show as "unavailable", "offline", or a variation of this depending on your hub. Without a battery back-up, there's no way for it to send a signal without power.
Usually the best way to monitor power outages is to take an old 12VDC power supply, cut the wire, and connect the two wires to a Z-Wave dry contact sensor.
These sensors almost always run on batteries and will report open/close circuits to the hub. Once the power supply is plugged in, the sensor will report status to the hub. If the power is cut to the supply, the sensor will also report this change to the hub.
If you have any questions about power outage status functionality, let us know!